In many applications, it is desirable to be able to move devices, such as machine tools, from one position to another. Often, such devices utilize DC torque motors for such purpose, the positioning of such motors being appropriately controlled by suitable control circuitry. While DC motors provide high acceleration and fast slew rates in producing the desired motion, one of the problems of using said motors is that, while they are controllable at high speeds, they become much more difficult to control at low speeds and are subject to sticking and slipping problems. Accordingly, it becomes relatively difficult to provide for precise positioning of the devices which are moved thereby.
An alternative approach to position control lies in the use of stepping motors, generally in an open loop configuration. In contrast with DC torque motors, stepping motors generally can be adequately controlled at relatively low speeds. However, it becomes difficult to obtain the relatively high accelerations needed for moving relatively long distances from one position to another since stepping motors tend to move in noticeably discrete steps, it being difficult to get such motors to rapidly slew to desired positions. When using stepping motors in an open loop configuration, for example, the acceleration, as determined by the change in the stepping rate must not be so great that the motor can "slip" back on its torque curve. If such slippage occurs, the exact position of the motor is lost and accurate tracking of motor position cannot be achieved. To avoid such slippage, the acceleration must be limited to a value much less than it potentially could be. Furthermore, stepping motors present resonance and ringing problems at very low speeds.
It is desirable, therefore, that control circuitry be devised for controlling the operation of a motor which can provide the relatively high acceleration and fast slew rates of DC torque motors while also providing the accurate control of stepping motors at relatively low speeds. It is also desirable to provide facilities for lessening the resonance and ringing problems at very low speeds.